1. Field of the Invention
The present invention relates to a method and apparatus for manufacturing corrugated paperboard, and more particularly, to a method and apparatus for injecting steam at the bonding nip of a single facer to accelerate the gelatinization of glue between a liner web and a medium web.
2. Description of the Prior Art
The manufacturing of double face corrugated paperboard typically begins with an apparatus known as a single facer. A conventional single facer includes an upper corrugating roll and a lower corrugating roll wherein each roll has a plurality of longitudinally extending teeth. The corrugating rolls are rotatably mounted adjacent each other such that the teeth of each roll are in a meshing relationship. A medium paperboard web is fed into a corrugating nip defined by the upper and lower corrugating rolls wherein the medium web conforms to the contour of the meshing teeth to form flutes in the medium web. Prior to entering the corrugating nip, the medium web typically passes over a preheater for increasing the temperature of the web.
The preheater typically comprises a steam pressurized drum heater having an internal cavity supplied with steam from an external source. The medium web is wrapped around a portion of the outer circumference of the drum and heat from the surface of the drum is transferred to the moving medium web.
A gluing roll, arranged to turn in a bath of starch-based glue, is positioned downstream from the corrugating nip and applies glue to the tips of the medium web flutes. The glue applied to the flutes of the paperboard webs is typically a suspension of raw or uncooked starch in a suitable liquid carrier. In this state, the starch has little or no adhesive qualities. However, at a certain temperature, dependent upon the type of starch utilized and the kind and amount of additives dissolved in the carrier, the starch granules will absorb the available liquid of suspension and swell, causing gelatinization of the suspension. In this gelatinized state the starch has superior adhesion abilities and will form a good bond between many substrates, including paper. The temperature at which gelatinization occurs for any particular formulation of glue can be easily determined by heating the particular formulation and observing the changes that occur in its viscosity.
As glue is applied to the paperboard medium web, a paperboard liner web is simultaneously supplied to a preheater having a design similar to that of the medium web preheater. Both the liner web and medium web preheaters depend on conduction for heat transfer to the respective paperboard web. Conduction heat transfer is directly related to the surface area of the paperboard web contacting the preheater, the duration of such contact and the temperature gradient between the preheater and the web. In order to provide sufficient heat transfer, the web preheaters must therefore define a relatively large surface area and the processing speed of the single facer must be limited.
The large surface area required of prior art preheaters substantially increases the overall size of the single facer. In fact, such preheaters are often so large that the preheater must be placed exterior to, and many times behind, the corrugating apparatus. Further, frictional forces opposing the movement of the liner and medium webs are substantially increased the greater the wrap angle around the outer surface of the preheater. Such frictional forces generate tension within the webs, often resulting in web breakage. Prior art attempts to eliminate such problems generated by friction have resulted in complex mechanical arrangements including rotatable preheater drums and variable wrap mechanisms.
The conventional single facer further includes a pressure roll arranged adjacent the lower corrugating roll for bringing the liner web into engagement with the glued flute tips of the medium web. The pressure roll and corrugating roll define a pressure nip for applying a pressure to the corrugated medium web and the liner web, whereby an initial bond is formed therebetween. While both webs are typically preheated to a predetermined temperature, additional heat is transferred to the webs by the pressure roll and lower corrugating roll. This combination of heat and pressure gelatinizes the glue between the medium web and liner web thereby forming a single face web of corrugated paperboard.
The pressure roll and corrugating roll are typically heated by high pressure steam passing through an internal channel. The high pressure steam heats the cylindrical walls of the rolls such that heat is transferred to the webs through conduction. This conduction heat transfer from the rolls is directly related to the surface area of the paperboard web contacting the roll, the duration of such contact, and the temperature gradient between the roll and the web.
Effective bonding of the medium web and liner web further depends upon the pressing duration and pressing force exerted by the pressure roll to force the webs together in an intimate relationship until a secure bond is formed by the gelatinization of the glue. The pressing duration is directly related to the length of the pressing nip and processing speed of the single facer. If the nip length is decreased while maintaining a constant processing speed then the pressing force must be increased to provide effective bonding. However, if the pressing force is decreased and the processing speed held constant then effective bonding necessitates that the nip length be increased.
Traditional pressure rolls provide a small nip length for acting against the medium web and the liner web, thereby necessitating a high pressing force. Such a high pressing force between the pressure roll and lower corrugating roll typically results in linear press marks, corresponding to the pitch of the teeth of the lower corrugating roll, being formed laterally on the surface of the liner web. Further, such a high pressure may weaken the liner web. The single face web produced by the prior art single facers therefore often have an unattractive appearance or are rejected as being defective.
In response to the above-noted problems, it has been proposed to replace the conventional pressure roll with an endless belt having a portion wrapped about the lower corrugating roll thereby forming an extended nip for pressing the liner web together with the medium web. More specifically, the endless belt is extended over a plurality of rolls to run freely in cooperation with the lower corrugating roll wherein the liner web and the corrugated medium web pass between the lower corrugating roll and the endless belt and are nipped therebetween.
A common problem associated with the prior art single facers employing such an endless belt is that the belt cannot provide sufficient pressing force given the available nip length to provide effective bonding of the liner web and the corrugated medium web. While the tension in the belt may be increased to thereby increase the pressing force, this tension is limited based upon the properties of the belt. Excessive tension in the belt may cause accelerated wear or tearing of the belt.
Accordingly, there is a need for a method and apparatus for providing an adequate bond between a medium web and a liner web to produce a single face web without damaging the liner web. Further, there is a need for such a method and apparatus for facilitating rapid gelatinization of the glue between the medium web and the liner web.